After the patch, a friend sent me note asking two questions about add-ons. He apparently thinks that, being a software developer by trade, I might have some insight into this topic. I've never created any add-ons myself, but here is my take on his questsions:
1) What is the incentive for someone to maintain extensive add-ons like Deadly Boss Mods, Auctioneer and X-Perl? There can't be a lot of money in it.
I suspect these started off a lot smaller than they are now. Some guy playing WoW had a need and decided to do something about it for himself. He's in a raid one day thinking, "I sure wish I could see everyone in the raid at once." So he sat down and hacked out an add-on that would do this for him. Then he sent it to a few friends, and they started using it, and started requesting that he add new features here and there. And he kept slowly tweaking it, adding things that he wanted or that someone else requested. Now five years later it is a gigantic behemoth with 7000 different configuration settings (give or take).
As to why he continues to maintain it… my guess is that it's just something he enjoys doing. It's just a hobby. People who can paint don't do it if they are only going to make money off of it. Amateur sculpters do so on their own time. Writing add-ons is just another type of hobby; it's a very creative process that can be personally rewarding for the right person. The fact there may also be thousands or even millions of people who use their work makes it that much more rewarding.
Not everything is about the money. (Just don't tell Gevlon.)
2) Why does so much stuff break every time Blizzard releases a new patch?
Let's see, how can I put this nicely? Blizzard has what we refer to in the software industry as "shitty development processes", "piss poor quality-assurance and testing", and an attitude of "we don't give a fuck regarding third-party developers".
As far as add-ons are concerned, there is absolutely no reason why Blizzard shouldn't be able to maintain backwards compatibility with 99.9% of the add-ons with each patch. But this would require actual planning and effort on Blizzard's part; which is apparently more than they are willing to invest in the development process. So instead, they just let everything break and leave it up to the independent developers to fix their own stuff, leaving everyone miserable in the interim.
From Blizzard's point of view, this is a smart business decision because (A) it costs them less money, and (B) they don't have to worry about customer satisfaction because we are all hopeless addicts and couldn't leave if we wanted to.